1. Field of the Invention
The present invention relates to methods of improving the pressure containment integrity of subterranean well bores containing drilling fluids or completion fluids.
2. Description of the Prior Art
In the drilling of wells (for example, oil and gas wells) using the rotary drilling method, drilling fluid is circulated through a drill string and drill bit and then back to the surface by way of the well bore being drilled. The drilling fluid maintains hydrostatic pressure on the subterranean formations through which the well bore is drilled to thereby prevent pressurized formation fluid from entering the well bore and to circulate cuttings out of the well bore. When the well bore reaches the top of the producing interval, a permeability damage reducing completion fluid is placed in the well bore and the producing interval is drilled using the completion fluid.
Once the well bore has been drilled to the desired depth, a string of pipe referred to as casing is positioned in the well bore. A hydraulic cement composition is pumped into the annular space between the walls of the well bore and the casing and allowed to set thereby forming an annular sheath of hardened substantially impermeable cement in the annulus. The cement sheath physically supports and positions the casing in the well bore and bonds the casing to the walls of the well bore whereby undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
The subterranean formations into or through which well bores are drilled often contain naturally occurring or drilling induced weak zones having low tensile strengths and/or openings such as natural fractures, faults and high permeability streaks through which drilling fluid is lost from the well bores or pressurized formation fluids enter the well bores. The drilling of additional well bores in producing fields often requires drilling through pressure depleted production zones that are weakened by pore pressures much lower than the original reservoir pressure. The weak zones in the well bores have low pressure containment integrity and are subject to failure as a result of the hydrostatic pressure exerted on them by drilling fluids or other treating fluids such as hydraulic cement slurries. That is, when a well fluid such as drilling fluid or a hydraulic cement slurry is introduced into the well bore, the combination of hydrostatic and friction pressure exerted on the walls of the well bore can exceed the strength of weak zones in the well bore and cause well bore fluid outflows into the formation containing the well bore. When the formation contains induced or natural formation fractures, faults or the like, well bore fluid outflows and/or pressurized formation fluid inflows, or both, can take place.
In addition, formation sands and shales having unexpected low well bore pressure containment integrity can be encountered while drilling. Thus, at any depth during the drilling or completion of a well bore, the well bore fluid circulating densities and pressures can exceed planned or designed densities and pressures. The excess pressure exerted within the well bore can and often does exceed the subterranean formation's well bore pressure containment integrity which causes outflow and loss of well bore fluids into the formation. Outflow pathways into the formation are opened over time (usually hours) to large dimensions that may contain losses many times the volume of the well bore fluids. Such losses can require substantial volumes of fluids to be pumped into the well bore in an attempt to maintain enough fluid column hydrostatic pressure to control pressurized formation fluids. Conventional plugging systems often fail to seal the outflow pathways and are also lost into the formation. In some cases, the loss rates may be higher than the pump-in rates causing lower fluid column heights in the well bore, reduced hydrostatic pressure below formation pore pressures and pressurized formation fluid inflow. In those cases, emergency measures are needed to contain the inflow at the surface and maintain well pressure control. Accordingly, when the first signs of poor well bore pressure containment integrity appear, rig operators are often forced to prematurely set casing or run a liner in the well bore. In many cases plugging back the well must be accomplished to allow casing to be set or to drill an adjacent sidetrack or bypass well bore. Each of these steps makes the overall cost of the well much higher than expected.
Thus, there are needs for reliable and quick methods of improving the pressure containment integrity of subterranean well bores during drilling.